A windmill for wind power generation is constructed in an area where humidity is high and temperature change is severe such as in a ridge of a mountain, seashore or the like. Therefore, when the temperature becomes low, moisture in the air becomes frost on a surface of a windmill blade and the adhered frost grows. Snow is easily adheres on the frost of the blade, and the snow near the edge of the blade causes a change of an angle of the blade. It causes a decrease of wind energy conversion efficiency.
In many cases, a blade of a windmill used for wind power generation is made of a fiber-reinforced plastic (FRP) and frost easily adheres on a blade made of FRP with no surface coating. The frost on the blade melts as the temperature goes up and the melt water re-freezes as the temperature goes down, thereby the adhered frost grows large and snow easily adheres thereon. As the windmill blade rotates when the frost adherence on the blade becomes low due to a rise of temperature, a part of the frost may scatter and causes a harm to humans or damages to structures.
There are similar problems to stated above with ice deposition which is a phenomenon that occurs by adhesion of ice as frozen water on a surface of an object and snow deposition which is a phenomenon that occurs by adhesion of snow on an object, and methods capable of suppressing the ice deposition and the snow deposition, and of easily removing the ice deposition have been proposed.
In order to improve the problems such as frost deposition and ice deposition, technology for reducing a contact angle between water and a member by performing a hydrophilization treatment on a surface of the member and for preventing water from staying on a surface of a member has been developed, and PTL 1 (JP-A-55-164264) discloses that synthetic silica and water-based coating material are used in combination.
PTL2 (JP-A-10-168381) discloses a high hydrophilic coating material using alumina sol instead of silica. In addition, PTL 2 also proposes formation of a film on a surface of a member using an organic surface treatment agent.
A coating composition in which a binder resin is mixed with a water-repellent fine particle is proposed as a coating composition used in a coating film. Examples of the binder resin of the coating composition include a solvent-soluble fluororesin, a silicone resin, a polyurethane resin, a polyester resin, and an acrylic silicone resin, and examples of the water-repellent fine particle include fluororesin powder (polytetrafluoroethylene, PTFE), silicone resin-based powder, water-repellent silica powder, graphite fluoride, and fluorinated pitch.
Provision of frost retarding properties or properties of easily removing frost on a surface of a structure in a cold region using a coating composition is realized by inhibiting adhesion of the frost or the like such that surface free energy on a surface of an object is reduced by forming a coating film excellent in water-repellency on the surface of the object, and that the surface of the object is reformed to be water repellent by reducing its affinity with water.
The terms frost, snow, and ice are applied differently depending on the environmental condition when water is transformed from a gas into a solid or from a liquid into a solid, and they are considered as the same as each other from the viewpoint of adhesion of water after being solidified on a surface of an object. Therefore, they are represented by frost hereinafter.